We propose to apply to an in vitro preparation of rat ventricular muscle, a novel method for separating the ion contents and concentrations of the sarcoplasm from those of the sarcoplasmic reticulum (SR), and for studying transsarcolemmal ion fluxes without interference from ion fluxes into and out of the SR. The method is based on the observation that sarcoplasmic ion content and composition remain stable while Na in the SR is lost when ventricles are equilibrated with isosmolal, NaCl-free sucrose Ringer's solution. Taking advantage of this separation, we shall study the Na-Ca exchange in the SR and the affinities and selectivity of the SR cation exchanger sites. Permeability of the SR will be examined by combining studies of penetration by labeled molecular probes of different sizes with a new morphometric analysis of the response of SR volume to solutions whose osmolality is varied by changing the concentrations of NaCl or sucrose. Since previous estimates of transsarcolemmal ion movements in mammalian heart muscle turn out to have been seriously distorted by failure to separate the contribution of the SR, these fluxes will be remeasured by the new technique, with particular emphasis on carrier-mediated transport mechanisms for Na and Mg and on passive transport of Cl. Stereologicaland other morphometric techniques will also be applied to perfused rat hearts to examine how the volumes of mitochondria and mitochondrial subcompartments respond to changes in extracellular osmolality and ion composition which produce defined changes in sarcoplasmic volume and ionic composition. In a separate project we propose to extend our previous morphometric analysis of membrane systems and other ultrastructures in normal and hypertrophied mammalian heart muscle to embryonic and neonatal hearts; the purpose of this study is to uncover the "laws" relating the growth of cardiac cellular membranes to the development of the electrical and contractile functions of heart muscle.